In some engines, an electronically controlled throttle is used for improved performance. In addition, engines may also be controlled using engine output, or torque control where the actual engine torque is controlled to a desired engine torque through an output adjusting device, such as with the electronic throttle, ignition timing, air-fuel ratio, or various other devices.
Also, engines using torque control and electronic throttles may also operate lean of stoichiometry to improve engine thermal efficiency and increase vehicle fuel economy. In these systems, the engine is operated lean of stoichiometry and relatively unthrottled, thereby decreasing engine pumping work and further increasing fuel efficiency. To increase the range of lean operation, direct injection can be used where fuel is injected directly into the engine.
Engines can also operate rich of stoichiometry. For example, rich operation may be required to purge stored emissions or to provide additional engine torque, as well as during various other situations.
One method to monitor engine control systems determines if actual engine torque is greater than desired engine torque. Actual engine torque is determined in accordance with engine airflow and/or injected fuel amount, and various other factors. Such a method is described in U.S. Pat. No. 5,692,472.
The inventors herein have recognized a disadvantage of the above approach. In particular, when such a method is utilized with engines that operate both lean of stoichiometry and rich of stoichiometry, differing torque estimates are produced depending on the operating state. Stated another way, when lean, engine torque determined from airflow is greater than engine torque determined from fuel injection amount. Similarly, when rich, engine torque determined from fuel injection amount is greater than engine torque determined from airflow. As such, engine torque may be over-estimated in one operating condition or another. Such over-estimation may degrade monitoring performance.